The experiments at the MacCHESS A1 beamline allowed us to collect x-ray diffraction data on crystals of two protein-protein complexes. One involves the cyclinA-CDK2 complex which was fully activated by phosphorylation, in the presence and absence of ATP (submitted); the other involves the complex of the p53 tumor suppressor bound to the 53BP2 protein which contains four ankyrin repeats and an SH3 domain. The crystals of the phosphorylated cyclinA-CDK2 complex, which diffract to 3.2 _ at our home source, produced 2.6 _ data at the A1 beamline. The 2.6 _ structure revealed that the phosphate group acts as an organizing center for CDK2 and induces conformational changes in the kinase. The phosphate group is mostly buried, and it is neutralized by three arginines which help extend its influence via a network of hydrogen bonds to both CDK2 and cyclinA. The conformational changes affect the putative substrate binding site as well as resulting in additional CDK2_cyclinA contacts. The crystals of the 53BP2-p53 complex produced data to 2.2 _ resolution which allowed us the unambiguous rebuilding of the complex (the crystals have a very high temperature factor) . The structure of the p53_53BP2 complex reveals that 53BP2 uses one of its four ankyrin repeats and its SH3 domain to bind the conserved surface of p53 that is also the site of DNA binding. The SH3_p53 interactions differ from previously characterized SH3_polyproline peptide complexes as the 53BP2 SH3 domain binds a rigid surface of p53 and not a polyproline helix. This structure also provides the first view of the ankyrin motif that is involved in protein-protein interactions in diverse regulatory molecules.